frogspawn

When first laid, frogs' eggs are small, pearlescent droplets a couple of millimetres across. They emerge from the female at a rate of one every few seconds, until she has laid 100 or more balls of jelly in the water. After a day or two they swell to double the size and the tiny black speck at the centre of each egg becomes visible.

Spring is a different matter. Between our front door and the public path there is a pond. It is only small: a couple of metres long by a metre wide and shallow enough that you can dredge the leaves from the bottom by hand. Modest it may be, but it supports a whole ecosystem of wildlife. My friend Simon found a leech in there one day and the children were thrilled to bits to know leeches live in their front pond. These leeches don't bite--at least they won't go for human blood, preferring to predate molluscs and other invertebrates.

Frogs, in common with most amphibians, can breathe through their skins, and this supplements the oxygen they can take in through a normal set of vertebrate lungs. Even though they don't have gills like fish, special pores in their skin allow them to take oxygen from the water, and remain submerged for extended periods. Some frogs even hibernate underwater, spending two or three months without taking a breath. In Spring, however, when three or four or more male frogs clasp the female in amplexus, and drag her under, they prevent the water from getting to her skin. So, as they pump their sperm, she gradually suffocates in their unbreaking embrace. It's not always like that, of course. That small pond out front has ten or twelve clumps of frogspawn. A dozen or more females made it through the sex. You sometimes see them, sitting on top of the pile of spawn, spitting tiny pearls out from her rear end as the male drenches them in sperm.

Researchers who study these things say that adult male amphibians will try to mount any suitably sized- and shaped-object, clasping it tight. If the object is a female ready to mate, then amplexus proceeds normally. However, if the object mounted is a female not ready to produce eggs, she will vibrate her body. When the clasping male feels the vibration, he stops clasping and leaves quickly. However, if there is no vibration, the male will stay clamped to the object, whether it is a dead female, a hairbrush or a lump of something unmentionable.

Our small domestic frogs have a delicate mating call. Something like a loud cat purr, but slower and deeper. Stand outside during the frog season and you can hear it, just louder than the background noise of birds: Brrrrrrp; Brrrrrrrp. It is a gentle, calming sound.

A week after the female has laid her spawn, the jelly has gone completely clear, and the pond appears deserted. There is just the occasional ripple on the surface as a frog hides among the fallen leaves that cover the bottom of the pond. The frogspawn floats just under the surface of the shallowest part of the pond. At the centre of each of a thousand eggs is a small, elongated nucleus. The beginnings of a tadpole.

Surrounding the nucleus is jelly. It provides all the nutrients the developing creature needs to develop into an independent froglet. Once the tadpoles hatch, the jelly provides food for all kinds of other pond-dwellers.

A week or two later, the eggs start to break down, and all that is left is a mass of brown-ish things that could be vegetation, or worms, or something else. In reality, they are young tadpoles. They remain almost immobile for a few days, after which they turn a darker colour and start swimming around in the water, looking for vegetation to eat.

A few days more and the vegetarian tadpoles turn carnivorous. In a healthy pond, there is plenty of food for the growing froglets, but if you put too much frogspawn into a clean tank, the tadpoles may end up cannibalising each other. Not a pretty sight for young children to watch. Adding small amounts of cat food to the tank water should keep the tadpoles from going hungry.